Many natural gas wells produce a relatively high pressure well stream effluent containing significant volumes of high vapor pressure condensates which will normally contain absorbed and dissolved natural gas, propane, butane, pentane and the like. Currently these liquid and dissolved hydrocarbons are only partially recovered by conventional, high pressure, separator units. The liquid hydrocarbon by-products normally removed from the well stream by a high pressure separator unit, are collected and then typically dumped to a low pressure storage tank means for subsequent sale and use. A substantial amount of dissolved gas and high vapor pressure hydrocarbons remain in the liquid hydrocarbon by-products. Substantial amounts of these gases and hydrocarbons may vaporize by flashing in the storage tank due to the substantial reduction in pressure in the tank which permits the volatile components to evaporate or off-gas into gas and vapor collected in the storage tank over the condensate. In this manner, substantial amounts of gas and entrained liquid hydrocarbons are often vented to the atmosphere to reduce storage tank pressure and are wasted. In addition to this initial vaporization and loss, further evaporation occurs when the condensate stands for a period of time in the storage tank or when the condensate is subsequently transported to another location or during subsequent storage and/or processing. This is described in the industry as weathering. Many users of the condensate specify particular low vaporization pressure requirements for such condensate; and the salability and value of the condensate depends upon the characteristics of the condensate.
Thus, natural gas wells, which produce significant amounts of high vapor pressure condensates along with the natural gas, present a great opportunity for improvement in production methods including a reduction in discharge to the environment and economic gain by recovery of otherwise wasted by-products. As previously described, present production equipment waste to the atmosphere large quantities of recoverable liquid and gaseous hydrocarbons, including absorbed and dissolved natural gas components. This waste occurs when the high vapor pressure liquid condensates and the dissolved gases are removed from the flowing gas stream by the separator, and through valving and sometimes intermediate pressure vessels, flashed when the pressure on the condensates is reduced to approximately atmospheric in the storage tanks.
One prior method directed at reducing the loss of liquid hydrocarbon components, which would otherwise be lost from flashing, has involved the use of a staging flash separator where the pressure of the condensate is reduced in stages. For example, the condensate pressure could be reduced in stages before transfer to a storage tank maintained at about atmospheric pressure.
Staging, in the manner described, can increase the recovered hydrocarbons by as much as 10% to 15%, but staging alone does not remove all of the absorbed gases and volatile hydrocarbon vapors from the condensate. The resulting liquid condensate still contains important components which, as previously described, cannot be completely held in the liquid phase at atmospheric pressure and will still be carried into the gases and vapors during flashing with the attendant loss of heavier entrained liquid hydrocarbon components of the condensate.
Another prior art method and apparatus for attempting to increase recovery of condensible hydrocarbons involves the use of very low temperature systems of the type disclosed by Maher U.S. Pat. No. 2,728,406. Such low temperature methods and apparatus depend upon chilling of a gas stream through pressure reduction to very low temperatures below the freezing point of water. Satisfactory operation of such low temperature systems have required use of antifreeze solutions to prevent freezing of liquids in the processing system. Furthermore, low temperature separation units cause a shrinkage of the volume and a reduction in the BTU content of the saleable natural gas, and unless pressurized liquid storage facilities are used, a low temperature separation unit will, in many cases, result in less rather than more liquid hydrocarbon recovery. The present invention does not employ any low temperature process nor low temperature apparatus of the type described in U.S. Pat. No. 2,728,406. To the contrary, the present invention employs relatively high temperature processes and apparatus wherein, under normal operating conditions, the temperature of the fluids being processed never falls below the freezing point of water (e.g., approximately 32.degree. F.) nor below gas hydrate formation temperatures of processed fluids.
It is, therefore, an objective of the present invention to provide an apparatus and method for more efficiently processing the additional recoverable gas and liquid hydrocarbon components normally contained in the condensates obtained from a natural gas wellhead gas-liquid separation system.